Explanation

Put a = 0.75. Since 1 + a + a² = (1 − a³)/(1 − a), the quantity under the root is a³/(1−a) + (1+a+a²) = (a³ + 1 − a³)/(1−a) = 1/(1−a) = 1/0.25 = 4, whose square root is 2 (option B).